Author: Nathaniel Tussing Greene

Publisher:

Published: 2012

Total Pages: 154

ISBN-13:

"Mammals localize sound along the horizontal plane principally using differences in the time and level of the sound present at the two ears. Evidence suggests that the lateral superior olive (LSO) initiates an excitatory pathway specialized to process interaural level differences (ILDs), and that the projections of LSO cells converge with additional ascending auditory pathways in the central nucleus of the inferior colliculus (ICC). Type I units in the ICC of decerebrate cats exhibit monaural and binaural response properties similar to those of LSO units, thus are supposed to be the midbrain component of an ILD processing pathway, but there have been no direct tests of this hypothesis. The goals of this thesis were to identify individual cells in the auditory midbrain whose responses are dominated by LSO influences, and re-evaluate the processing of complex sounds within this pathway. Most prior studies of LSO have been conducted under the influence of anesthesia, which can dramatically alter the balance of excitation and inhibition in the brainstem. The results of direct comparisons in decerebrate cat suggest that the responses of LSO units are consistent with providing the dominant excitatory input to ICC type I units, but that additional excitatory and inhibitory inputs result in hierarchical transformations in the ICC. Pharmacological inactivation of LSO while recording ICC units reveals that at least half of ICC type I units do indeed derive their principal excitatory drive from LSO, compared to few or no type V or O units. These results suggest that a functional segregation of LSO projections is maintained in the auditory midbrain. Processing of sounds within this pathway is then assessed by comparing the responses of LSO and ICC units to amplitude modulated (AM) tones, which are a prominent component of speech and other behaviorally important stimuli. The responses recorded in LSO suggest that AM sensitivity is enhanced at low levels compared to its inputs, and comparisons to ICC type I unit responses reveal additional improvement. These results suggest a previously unidentified role for the LSO to ICC type I pathway in the processing of AM stimuli. In conclusion, this thesis demonstrates that type I units are the midbrain components of an ILD processing pathway, and that additional inputs shape the responses of cells in this pathway. These results support the hypothesis that pathways initiated by some brainstem nuclei remain functionally segregated in the midbrain, and their responses are transformed via hierarchical processing"--Pages iv-v.